This invention relates generally to electrostatic latent image development systems that operate using liquid developing material, and, more particularly, relates to a system for electrostatic development of a latent image, wherein the latent image is developed with use of a toner cake layer having a high solids content toner.
Various methods of developing a latent image have been described in the art of electrophotographic printing and copying systems. A typical electrostatographic printing process includes a development step whereby a quantity of developing material is physically transported into the vicinity of a latent image bearing imaging member, with the marking particles (described herein as toner) in the developing material are caused to migrate via, e.g., electrical attraction, to the image areas of the latent image so as to selectively adhere to the imaging member in an image-wise configuration.
Of particular interest with respect to the present invention is the concept of forming a thin layer of liquid developing material on a first surface of a first member, wherein the layer has a high concentration of charged toner. The layer on the first member is brought into contact with an electrostatic latent image on a second surface of a second member, wherein development of the latent image occurs upon separation of the first and second surfaces, as a function of the electric field strength generated by the latent image. In this process, toner particle migration or electrophoresis is replaced by direct surface-to-surface transfer of a toner layer induced by image-wise fields.
For the purposes of the present description, the concept of latent image development via direct surface-to-surface transfer of a toner layer via image-wise fields will be identified generally as contact electrostatic printing (CEP). Exemplary patents which may describe certain general aspects of contact electrostatic printing, as well as specific apparatus therefor, may be found in U.S. Pat. Nos. 4,504,138; 5,436,706; 5,596,396; 5,610,694; and 5,619,313.
It is desirable that the aforementioned layer of liquid developing material be provided in a very thin and very uniform layer that exhibits a high proportion of solids, that is, having a high solids content. Even more desirable is such a layer exhibiting the following advantageous characteristics: a selectable, uniform thickness, preferably in the range of 3-10 microns; a high solids content, preferably in the range of 15 to 35 percent solids; and an accurately metered mass per unit area on the order of 0.1 mg per cm2.
The intuitive and conventional approach is to attempt the formation of such a layer by direct application of liquid developing material having a high solids content. However, due to the very complicated rheological behavior of a liquid developing material having the requisite high solids content, such direct application of a supply of such liquid developing material to a receiving member typically does not achieve a layer having the aforementioned desirable characteristics. For example, the resulting layer has been found to exhibit a variable thickness and a non-uniform mass per unit area, which renders the layer generally unsuitable for most contact electrostatic printing applications.
In accordance with one aspect of the present invention, there is provided an imaging system for effecting electrostatic printing of an image, wherein the imaging system includes at least one contact electrostatic printing engine operable in a novel fashion upon a copy substrate, wherein each contact electrostatic printing engine images and develops an electrostatic latent image representative of the image, and subsequently transfers the developed image to the copy substrate.
In accordance with another aspect of the present invention, a toner cake delivery apparatus may be constructed and operated in accordance with the contact electrostatic printing process to which the present invention is directed, wherein a secondary toner cake layer of high solids content and selectable, uniform thickness is created from a primary toner cake layer. This secondary toner cake layer is then highly advantageous for use in development of an electrostatic latent image.
In one embodiment of the present invention, a primary toner cake layer is created on a donor surface of a movable coating member and is subject to uniform charging to a first polarity. Next, the primary toner cake layer is exposed to a charging device whereupon it is subject to an induced charge of a second, opposing polarity to a selectable depth. As a result, the primary toner cake layer exhibits, when considered along its cross-sectional dimension, an inner layer having uniform charge of the first polarity and an outer layer having uniform charge of the second polarity. The primary toner cake layer is brought into pressure contact at a process nip with a receiving surface of movable receiving member. The donor surface and the receiving surface are subject to a electrical bias differential such that the outer layer is subject to a strong attraction to the receiving surface. Due to the bias differential and subsequent separation of the donor surface and the receiving surface, the primary toner cake layer is divided. The outer layer of the primary toner cake layer transfers to the receiving surface of the receiving member and the inner layer of the primary toner cake layer remains on the donor surface. At least one of the inner and outer layers is generally characterized as having a high solids content (e.g., approximately 10-50 percent solids, and preferably in the range of approximately 15 to 35 percent solids, or greater), and exhibits the additional advantageous characteristics of a uniform thickness, in the range of 1-15 microns, and an accurately metered mass per unit area on the order of 0.1 mg per cm2. In a preferred embodiment, the outer layer of the primary toner cake layer is used as the desired secondary toner cake layer, and the inner layer of the primary toner cake layer remains on the donor surface as a residual toner cake layer.
In another embodiment of the present invention, a primary toner cake layer is created on a receiving surface of a receiving member and is subject to uniform charging to a first polarity. Next, the primary toner cake layer is exposed to a charging device whereupon it is subject to an induced charge of a second, opposing polarity to a selectable depth. As a result, the primary toner cake layer exhibits, when considered along its cross-sectional dimension, an inner layer having uniform charge of the first polarity and an outer layer having uniform charge of the second polarity. The primary toner cake layer is brought into pressure contact at a process nip with a donor surface of coating member. The donor surface and the receiving surface are subject to a electrical bias differential such that the outer layer is subject to a strong attraction to the receiving surface. Due to the bias differential and subsequent separation of the donor surface and the receiving surface, the primary toner cake layer is divided. The outer layer of the primary toner cake layer transfers to the donor surface of the coating member so as to form a residual toner cake layer and the inner layer of the primary toner cake layer remains on the receiving surface so as to form a secondary toner cake layer.
In an alternative to the foregoing embodiments, the charging device may be omitted and the applied bias differential is controlled so as to reverse the first polarity charges only to a selectable depth. As a result, the primary toner cake layer exhibits, when considered along its cross-sectional dimension, an inner layer having uniform charge of the first polarity and an outer layer having uniform charge of the second polarity.
In another alternative embodiment, the toner cake delivery apparatus accomplishes the aforementioned uniform charging to a first polarity by use of a supply of high solids content liquid developing material generally made up of toner particles immersed in a liquid carrier material and also typically including a charge director for providing a mechanism for producing an electrochemical reaction in the liquid developing material composition which generates the desired electrical charge on the toner particles. A high solids content liquid developing material applicator provides a relatively uniform, charge layer of high solids content liquid developing material onto the coating member, which may be provided in the form of a drum, web, belt, or similar coating member.
An embodiment of a contact electrostatic printing engine may be constructed to include the aforementioned toner cake layer delivery apparatus and an imaging member for receiving an electrostatic latent image. The imaging member acts as the aforementioned receiving member and thus includes an image bearing surface capable of receiving and supporting the aforementioned secondary toner cake layer. An image-wise exposure device is provided for generating the electrostatic latent image on the imaging member, wherein the electrostatic latent image includes image areas defined by a first charge voltage and non-image areas defined by a second charge voltage distinguishable from the first charge voltage. The aforementioned toner cake delivery apparatus is then employed for depositing the secondary toner cake layer on the image bearing surface of the imaging member so as to form a layer of high solids content that is adjacent the electrostatic latent image on the imaging surface of the imaging member. A separator member is also provided for selectively separating portions of the secondary toner cake layer in accordance with the latent image, to create a developed image corresponding to the electrostatic latent image formed on the imaging member.